1. Field of the Invention
The present invention relates to an electric double layer capacitor.
2. Prior Art
The energy that can be stored in an electric double layer capacitor is proportional to the square of the voltage applied to the capacitor. Theoretically, it should be possible to increase the stored energy of the capacitor by increasing the voltage applied to the capacitor.
However, in conventional electric double layer capacitors which use organic electrolytic solutions, when a relatively high voltage of about 2.5 V or more is applied, both the positive and negative electrodes reach their reaction potentials, giving rise to decomposition of the electrolytic solution, and thus leading to problems such as declines in the durability and cycle characteristics of the capacitor.
In light of this, efforts are being made to find ways of increasing the voltage rating of capacitors within a range where the respective reaction potentials (decomposition potentials) of the positive and negative electrodes are not reached by controlling the capacitance ratio between the positive and negative polarizable electrodes within an appropriate range.
For example, JP-B 6-65206 discloses an electric double layer capacitor in which, by adjusting the capacitances of the positive and negative polarizable electrodes to a ratio which is the reciprocal of the ratio between the reaction potentials of the positive and negative electrodes, with reference to 0 potential in a plot of the potential-current characteristics, the time it takes for the positive and negative electrode to reach their respective reaction potentials when a voltage is applied can be made the same.
JP-A 8-107047 discloses an electric double layer capacitor in which the specific surface area of the activated carbons used in the positive and negative polarizable electrodes and the weight ratio of the activated carbons are controlled within suitable ranges.
JP-A 9-92583 discloses an electric double layer capacitor in which, by controlling the coating weight ratio in the positive and negative polarizable electrodes, the capacitance ratio between the positive and negative polarizable electrodes is set in accordance with the respective potentials at which an irreversible current arises in the positive and negative electrodes.
JP-A 10-270293 discloses an electric double layer capacitor in which the capacitances of the respective positive and negative electrodes are made to differ by, for example, varying the surfaces areas of the positive and negative polarizable electrodes.
JP-A 2000-188244 discloses an electric double layer capacitor in which the capacitance ratio between the positive electrode and negative electrode used as the polarizable electrodes is 1.5 or higher.
JP-A 2003-289022 discloses an electric double layer capacitor in which the solid volume or weight distribution of the positive and negative polarizable electrodes has been optimized.
It is known that, in conventional electric double layer capacitors which use organic electrolytic solutions, when a high voltage is applied, the positive electrode side reaches the reaction potential first.
The solution employed in the above and other capacitors already known to the art is to make the coating weight of the activated carbon on the positive electrode side higher than that on the negative electrode side so as to increase the capacitance of the positive electrode and lower the potential of the positive polarizable electrode, thereby delaying arrival of the positive electrode side at the reaction potential.
However, control of the positive and negative electrode potentials is transient. It is not always possible, merely by setting the capacitance ratio or the coating weight ratio of the positive and negative polarizable electrodes to a value that represents an excess of the positive electrode, to obtain a capacitor having excellent durability and cycle characteristics.
JP-A 11-67608 discloses an electric double layer capacitor in which activated carbons of mutually differing pore size distributions are used in the positive and negative polarizable electrodes.
However, this disclosure makes no mention of any association with the electrolyte used, nor does it suggest or imply anything specific concerning the combination of positive and negative pore size distributions and the capacitor characteristics. Furthermore, the coating weights of the activated carbons used in the positive and negative electrodes are the same.
In any case, there have hitherto been no known electric double layer capacitors which, when continuously charged under the application of a high voltage of about 3V, clearly suppress a decrease in capacitance and a rise in internal resistance, and thus have an excellent durability.